It is common oilfield practice to emplace a string of coil tubing in a completed oil or gas wellbore. The coil tubing may be used for any of a variety of purposes. For example, it can be used for chemical injection; steam injection, service as a bubble tube for measuring downhole pressure, sand clean-out, solvent spotting or circulation, or for carrying sensor instrumentation for measuring downhole pressure or temperature. The present invention was developed in connection with using coil tubing for carrying sensor instrumentation downhole.
Coil tubing has a relatively small diameter. The diameter can vary between 1/8 inch and 2 inch. As a result, a string of coil tubing is pretty flexible. It has a tendency to spiral or corkscrew as it is being run into a wellbore. Also, it does not lend itself to being forced through wellbore tight spots, sand plugs and the like. These problems are magnified when trying to run the coil tubing in to the toe end of a horizontal wellbore, such as a wellbore to be used in a steam-assisted gravity drainage ("SAGD") project. (A horizontal wellbore usually extends downwardly from ground surface to a hydrocarbon-containing reservoir and then bends to extend, generally horizontally, into the reservoir. The wellbore is described as having a "heel" (at the bend) and a "toe" (at the far end of the wellbore). The section of wellbore between heel and toe is often referred to as the "production interval").
As previously indicated, it is known to emplace a coil tubing instrumentation string in the horizontal section of a wellbore to measure formation temperature or pressure and relay this information to ground surface through cable means. From this information, the operator can develop a temperature or pressure profile extending the length of the production interval. However this requires that the string extend from the heel to the toe of the horizontal wellbore section.
So one problem to be addressed is how to better insert a coil tubing string to the desired landing point in a wellbore. In the case of a wellbore having a horizontal production interval, the problem is how to better insert the string so that it reaches the toe end of the wellbore. However, the solution to the problem needs to ensure that, once the coil tubing string is in place downhole, it is present as a freely and independently movable string.